A line dresses are more of a cutting trick than a sewing trick. Measure your
doll from her neckline to wherever you want the dress to fall, and add a
half inch.

Fold your cloth. Place your bodice pattern on the edge. The length between
the top of the neckline and the edge of the cloth should equal that measurement
you just took. Leave space behind the pattern!

Now, cut your cloth, but when you get to the armpit, cut diagonally
away from the bodice to the edge of the cloth. Here, have an MSPaint diagram: